Okay so, in my superhero setting, the way humanity gets superpowers is due to a single mutation. People rarely had superpowers before due to the Dragon gene, but it was always a recessive trait and being Dragonbreed meant you were infertile.
However at some specific point in history, a person named Kurt Snow - also an adventurer named The Claw - is born with a mutation that allows him to easily pass on the Dragon gene. And since he's immortal and prone to romances, he basically single-handedly passes his mutated genes to human population over the course of next couple centuries. I imagine he might have somewhere like 500-1000 children, but by modern 21st century superhumans are relatively common and huge chunk of people have that gene, active or not.
So... this kind of math is something that's too much for me to wrap my head around. Basically I'm asking is in what century does Kurt Snow need to be born for it to be reasonable that this mutated gene would become somewhat common in modern population?